It was pretty simple, step by step pictures attached (Have to be logged in to see them). Battery is a single cell lithium ion. Any 3.7v single cell lion or lipo will work. The white plastic connectors are called 2.54mm JST, the wire is 24 gauge stranded, and all I did was solder the connector wires onto the PCB where the red and black wires from the battery were already attached. I've been leaving the extra battery attached while charging the JXD and no problems so far. Takes a lot longer to charge, but seems to last 8-10+ hours of solid use too.
Only trouble I can see anyone running into, is I've seen a picture of a disassembled jxd and it didn't look like the internal battery was wired into the same place on the PCB. So if there are a couple different revisions out there and the one you have is different then you might have to pull it further apart to figure out where the battery is actually connected. Oh, and the battery indicator doesn't really work, it just sits at 99% for quite a few hours and then starts slowly going down but being a jxd the battery indicator never really worked before either.